Biaxially oriented, heat shrinkable cook-in casings made from a multilayer film containing polyamide or polyester have been known for some time. Furthermore, various blends comprising polyamides and other polymers are also known for use in such cook-in casing films.
A commercially available casing used for cook-in applications has included a layer of ethylene/vinyl alcohol copolymer (EVOH), present as an O.sub.2 -barrier layer, this layer being outward of one or more film layers comprising one or more polyolefins. Such films have been found to be orientable and shirrable without splitting during cooking.
EVOH is known to be an expensive copolymer. Moreover, in certain end uses, such as cook-and-strip end uses, it is not necessary to provide an O.sub.2 -barrier layer, as the oxygen permeability during the cook-in step is not important. Thus, for cook-and-strip end use, it is desired to provide a cook-in casing which does not contain the relatively expensive EVOH layer. However, upon elimination of the EVOH layer, the inventors have discovered that a substantially 100% polyolefin casing tends to split during cook-in. It has been theorized that the reason for this splitting is that oil, which is placed on the outside of the casing during the shirring process, migrates into the film and weakens the polyolefin layers. It is believed that the EVOH layer prevented the migration of the oil to the polyolefin layers inward of the EVOH layer, and for this reason the EVOH-containing casings do not tend to split during cook-in. Nevertheless, such EVOH containing casings remain expensive and potentially unnecessary for cook-in applications, provided some way can be found to eliminate the detrimental effects of the oil on the polyolefin layers.
In an attempt to overcome the casing-splitting problem while at the same time eliminating the use of expensive EVOH in the casing, the instant inventors conceived of using a polyamide layer in place of the EVOH layer, the polyamide layer being between the oil and the polyolefin layer(s), in order to block the migration of oil and thereby reduce or eliminate the detrimental effects of the oil on the polyolefin layer(s). However, nylon 6, nylon 66, and nylon 6/66 are about the only feasible polyamides for such use, as the remaining polyamides are as expensive, or more expensive, than EVOH.
The inventors discovered that a film layer of 100% nylon 6 cannot be oriented to the degree required to provide the casing with a desirable level of heat-shrinkability, as the orientation bubble would break very frequently, rather than orient. Thus, there remains a need for an EVOH-free, relatively inexpensive article suitable for cook-in end use.